The present invention relates to a continuous, two-stage process for complete dehydration and subsequent calcination of aluminum fluoride hydrates (e.g. AlF.sub.3.3H.sub.2 0), to obtain a high yield of a product of especially high purity. The invention relates in particular to a two-stage process in which the second stage comprises the final dehydration of the semi-hydrate (AlF.sub.3.0.5H.sub.2 0) and its calcination.
On crystallizing AlF.sub.3 from an aqueous solution, hydrated forms of this fluoride are obtained, amongst these aluminum fluoride trihydrate.
The dehydration of this hydrate is problematic because, during the heating of the hydrate to the calcination temperature of 550.degree.-600.degree. C., a hydrolysis reaction takes place in contact with the water vapor and, with the formation of HF, Al.sub.2 O.sub.3 is formed. EQU 2AlF.sub.3 +3H.sub.2 0.fwdarw.Al.sub.2 0.sub.3 +6HF
If aluminum fluoride hydrate is calcined in counter flow in a directly heated, lined rotary fumace, the resultant calcined product contains only 86-92% at max. of AlF.sub.3 the remainder being essentially Al.sub.2 O.sub.3. The German Pat. DE-PS No. 1 166 754 describes a process for the calcination of the AlF.sub.3 hydrate in a fluidized bed to give calcined aluminum fluoride, at the same time eliminating the hydrolysis reaction. A special feature of this process is that the hydrate is heated to the calcination temperatures in question in less than one to a few seconds. This causes the water of hydration to be dissociated and removed quickly to avoid re-hydrolisation reactions. In large scale production this process leads to 96% calcined AlF.sub.3, and represents therefore a substantial improvement over direct calcination.
The device for carrying out the process of DE-PS No. 1 166 754 for continuous calcination of AlF.sub.3 hydrates is described in the patent AT-PS No. 28 533. The apparatus described there functions satisfactorily on a small scale. However, if it is to be applied in large capacity plants, it turns out to be relatively complicated, not always easy to control and requires expensive supervision and maintainance. There was therefore a great need to find a solution which is simpler from the technical viewpoint.
The Swiss Pat. CH-PS No. 580 033 has therefore already described a device for the calcination of AlF.sub.3 tri-hydrate to the semi-hydrate which calcine AlF.sub.3 hydrate to AlF.sub.3 semi-hydrate at calcination temperatures of 190.degree.-200.degree. C. on a large scale in a plate type dryer. Using this calcination temperature and a subsequent final calcination in a fluidized bed, calcined AlF.sub.3 with an AlF.sub.3 content of 96% is also obtained. It is however a disadvantage that in this case too the final calcination temperature for the AlF.sub.3 must be reached within a few seconds.
From the state of the art it can be seen that relatively complex processes and/or devices have been employed to solve the problems arising from the dehydration of AlF.sub.3 hydrates.
The object of the invention is therefore to simplify the process and, without reducing the yield and sacrificing the high purity of the product, at the same time improving the reliability of the operation.